Living (and Sometimes Dying) with Karst

The earth opened up and swallowed Jeff Bush last week. Normally, I wouldn't use that phrase: people say it all the time when the earth has done no such thing. But in this case, it fits. A man went to his bedroom. The land fell away beneath him, and collapsed in on him, and he died.

We like to make sense of these events. What happened to Jeff Bush was random and rare, but the fact of the earth falling away beneath us is all too common. In the United States alone, over a fifth of the place is karst country, where sinkholes can happen, and often do. If we zoom in on Florida, we notice something rather striking: pretty much all of it is susceptible to sinkholes.

What is karst? In simple terms, it's what you end up with after rocks spend a lot of time dissolving. It's full of caves and sinkholes, solution valleys and underground streams. It's quite common in places where the bedrock is mostly limestone, dolomite, or gypsum. It can even happen in places where marble and salt (yes, salt) are major constituents. Anything that can be dissolved by weak carbonic acid can become karst country. And what's weak carbonic acid? Water that's mixed with carbon dioxide, simply. You can watch it at work in old limestone statues adorning exposed areas on buildings. This gargoyle once had a less-melty face, for instance: acid rain that wouldn't hurt a human plays merry hell with the calcite in limestone.

Now imagine that happening on a huge scale to the landscape at large, and you've got an idea what causes karst. Then keep in mind that pretty much all of Florida is limestone, subject to dissolution.

What we see of Florida is just part of an enormous carbonate platform that developed over millions of years. When the bit of metamorphic rock that would become Florida's basement rifted off of proto-Africa in the time of Pangea, a basin formed as the baby Atlantic Ocean was born. That's where the Florida Platform began to form: in shallow, warm seas filled with coral reefs and algae with calcium carbonate skeletons, living happily under the sun, producing those calcium carbonate bits that built up incrementally as they died. And this went on for ages, right up through the Paleogene, the geologic age after the death of the dinosaurs. There are cliffs bounding the Florida Platform that are nearly 1,828 meters (6,000 feet) high, beneath the waters of the Gulf of Mexico. That immense carbonate mass may be more than 6,096 meters (20,000 feet) deep. That's a huge amount of potential karst.

Millions of years into the Paleogene, the Appalachians were uplifted in one of the occasional orogenies that tend to happen when plates are moving around, and the material eroded off of them was carried down by longshore currents and rivers. Those sediments included the quartz sands that became Florida's famous beaches, and the silts and clays that form a relatively thin cover to the limestone bedrock.

And that limestone has been subject to dissolution by slightly acidic ground- and rain-water for millions of years. It's lousy with caves, crevices, underground passages, and other hollow bits. That spells sinkholes.

Sinkholes come in a variety of shapes, sizes and mechanisms of formation, but the one that opened beneath Jeff Bush's bedroom was most likely a cover-collapse sinkhole. Picture a cave, a void in the limestone, that's been growing for some time as weakly acidic water gradually eats the stone away. Gravity works on hollow spaces: collapse is natural. The ceiling becomes too weak to stand, and caves in. Sometimes, if the overlying soil is 9-61 meters (30-200 feet) thick, and made mainly of clay, the ground doesn't open right away. The sturdy clay bridges the gap. But it's only so strong, and things happen. Perhaps it gets too wet, or, oddly enough, too dry - when it dries, it crumbles. And then it comes down, with whatever's grown or been built atop it, in a matter of minutes or hours.

And this is what we have to live with, in karst areas: the possibility that solid land isn't so solid as we think, and may give way with little warning, swallowing homes and vehicles and crops and, in extremely rare instances, people.

We can mitigate the hazard. We know some of the things that cause sinkholes to form. Pumping out too much groundwater is a common one: messing about with the drainage and surface water is another. Drilling wells, dewatering foundations, erecting buildings; leaky pipes or septic tanks, drainage ditches, impoundment of surface water in ponds - all of these things can lead to sinkholes. So it pays to pay attention, and be very careful indeed. Before an area is built upon, geologists should have a thorough look, and their assessments be taken seriously. Water tables shouldn't be pumped below a certain minimum depth, dependent upon the area, to maintain the buoyant support holding up the overlying land. And if you're living in karst country, pay attention to these warning signs:

Fresh exposures on fence posts, foundations and trees that result when the ground sinks.

Slumping, sagging or slanting fence posts, trees or other objects; doors and windows that fail to close properly.

Ponding — small ponds of rainfall forming where water has not collected before.

Wilting of small, circular areas of vegetation because the moisture that normally supports vegetation in the area is draining into the sinkhole that is developing below the surface.

Turbidity in water in nearby wells during early stages of sinkhole development.

Structural cracks in walls, floors and pavement; cracks in the ground surface.

Sudden draining of a pond or creek.

Small conical holes that appear in the ground over a relatively short period of time.

There are ways to fix sinkholes so that they don't continue to grow and threaten other structures, but if you have a sinkhole on your property, don't attempt to fill it on your own. Hire a professional.

I wish I could tell you that sinkholes were a freak of nature that rarely happen, but as we encroach on karst country and mess about with it without due care, they're going to become more frequent. As areas get built up, they'll affect people more, just like any other natural process that people manage to accelerate. That means we'll have to understand them, and work to mitigate them. There are ways of living with karst, never perfectly, but better than barging in on it without understanding its challenges.

Hopefully, with due care and attention, and a little bit of luck, we can prevent the earth from swallowing people without warning.

Veni, George et al (2001): Living with Karst: A Fragile Foundation. An excellent book on the geology and hydrology of karst regions, complete with sound suggestion on hazard mitigation. It's written in clear, concise language with technical terms clearly explained. I recommend it for anyone living in karst country.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Dana Hunter

A confirmed adorer of the good science of rock-breaking, Dana Hunter explores geology with an emphasis on volcanic processes, geology news, and the intersection of science and society. Her home away from SciAm is Dana Hunter's Unconformity

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